1. Field of the Invention
This invention relates to a control system for an internal combustion engine, and more specifically to a control system for an internal combustion engine having a plurality of cylinders and a cylinder halting mechanism for halting some of the cylinders.
2. Description of the Related Art
Japanese Patent Laid-Open No. Sho 62-250351 discloses a method for detecting an abnormality of an oxygen concentration sensor provided in an exhaust system of an internal combustion engine. According to this method, an abnormality of the oxygen concentration sensor is detected based on an output of the sensor during the fuel-cut operation in which fuel supply to the engine is stopped.
Further, Japanese Patent Laid-Open No. 2001-234792 discloses an internal combustion engine having a cylinder halting mechanism. By means of the cylinder halting mechanism, a partial-cylinder operation in which some of the plural cylinders are halted, and an all-cylinder operation in which all of the cylinders are operating are switched according to the operating condition of the engine. Specifically, the engine disclosed in Japanese Patent Laid-Open No. 2001-234792 is a V-type six-cylinder engine having a right bank and a left bank each of which includes three cylinders. When the engine is operating in a low load condition, operation of intake valves and exhaust valves of the three cylinders on the right bank is halted.
If the abnormality detection method disclosed in Japanese Patent Laid-Open No. Sho 62-250351 is applied as it is to an oxygen concentration sensor mounted on the engine disclosed in Japanese Patent Laid-Open No. 2001-234792, the following problem arises.
An oxygen concentration sensor is provided in an exhaust system of the engine in order to perform a feedback control of the air-fuel ratio. In one example of a V-type six-cylinder engine, two oxygen concentration sensors are disposed corresponding respectively to the right bank and the left bank. In this instance, when performing the partial-cylinder operation, operation of the intake valves and exhaust valves of the right bank is stopped. Consequently, no exhaust gas flows through the exhaust pipe on the right bank, but exhaust gases exhausted immediately before the valve stoppage stay in the exhaust pipe. As a result, the oxygen concentration sensor does not detect a high oxygen concentration which is to be detected during the fuel-cut operation, to thereby make a wrong determination is made that the oxygen concentration sensor is abnormal.
It is an object of the present invention to provide a control system for an internal combustion engine, which can accurately determine a failure of an oxygen concentration sensor mounted on the internal combustion engine whose operation is switched between the partial-cylinder operation and the all-cylinder operation.
The present invention provides a control system for an internal combustion engine (1) having a plurality of cylinders (#1-#6) and switching means (30) for switching between an all-cylinder operation in which all of the cylinders is operated and a partial-cylinder operation in which at least one of the plurality of cylinders is halted. The control system includes operating parameter detecting means (4, 8-10, 15, 16), instructing means, an oxygen concentration sensor (22R), diagnosing means, and permitting means. The operating parameter detecting means detects operating parameters of a vehicle driven by the engine. The operating parameters include at least one operating parameter of the engine. The instructing means instructs the switching means (30) to perform the all-cylinder operation or the partial-cylinder operation according to the operating parameters. The oxygen concentration sensor (22R) is provided in an exhaust system (13R) corresponding to the at least one cylinder (#1-#3) which is halted during the partial-cylinder operation, and detects an oxygen concentration in exhaust gases. The diagnosing means diagnoses a failure of the oxygen concentration sensor (22R) in a predetermined operating condition including a fuel-cut operation of the engine upon deceleration. In the fuel-cut operation, fuel supply to the engine is stopped. The permitting means permits the partial-cylinder operation after completion of the failure diagnosis by the diagnosing means.
With this configuration, the failure diagnosis of the oxygen concentration sensor provided on the exhaust system of the engine is performed in the predetermined operating condition including fuel-cut operation upon deceleration of the engine, and partial-cylinder operation is permitted after completion of the failure diagnosis. Accordingly, the failure diagnosis of the oxygen concentration sensor is first performed during the all-cylinder operation, and the partial-cylinder operation is made executable after completion of the failure diagnosis. Therefore, a failure of the oxygen concentration sensor mounted on the halted cylinder side can be diagnosed accurately.
Preferably, the engine has a first bank including a plurality of cylinders (#1-#3) and a second bank including a plurality of cylinders (#4#6), and the plurality of cylinders (#1-#3) on the first bank are halted during the partial-cylinder operation.
Preferably, the engine has a first exhaust pipe (13R) connected to the first bank and a second exhaust pipe (13L) connected to the second bank, and the oxygen concentration sensor (22R) is disposed in the first exhaust pipe (13R).
Preferably, the diagnosing means determines that the oxygen concentration sensor (22R) fails, when an output (SVO2) of the oxygen concentration sensor indicates a rich air-fuel ratio immediately after starting of the fuel-cut operation, and the output (SVO2) of the oxygen concentration sensor still indicates a rich air-fuel ratio after a first predetermined time period (TMMODE2) has elapsed from the starting of the fuel-cut operation.
Preferably, the diagnosing means determines that the oxygen concentration sensor is normal, when an output of the oxygen concentration sensor indicates a lean air-fuel ratio immediately after starting of the fuel-cut operation, and the output of the oxygen concentration sensor changes to a value indicative of a rich air-fuel ratio within a second predetermined time period (TMMODE3) after the fuel cut operation ends.